1. Field of the Invention
The present invention relates to a method of manufacturing a micro-tip for detecting tunneling current, micro-force or magnetic force, a female mold substrate for the manufacture thereof, a method of manufacturing a probe with such a micro-tip, a probe unit, a scanning probe microscope and an information recording/reproduction apparatus having such a probe.
2. Related Background Art
In recent years, a scanning tunneling microscope (hereinafter abbreviated as an "STM") that allows direct observation of the electron structures of atoms on conductor surfaces has been developed (G. Binnig et al., Phys. Rev. Lett., 49, 57 (1982)), and a real space image can now be measured at a high resolution, irrespective of whether single-crystal or amorphous materials.
In addition, such scanning tunneling microscope has an advantage of permitting observation with a low electric power without damaging the specimen tending to be damaged by current, and of being applicable in the open air and to various materials. It is therefore expected to be applied in a wide range of uses.
Such STM utilizes a tunneling current that flows across a metal tip and a conductive material when a voltage is applied across them and the tip is brought close to a distance of about 1 nm to the conductive material.
This current is very sensitive to a change in distance between the tip and the conductive material, and changes exponentially. Accordingly, by scanning the tip to maintain a constant tunneling current, the surface structure of a real space can be observed at a resolution on the atomic order.
Objects that can be analyzed using the STM are limited to conductive materials. Recently however, the STM is often applied to structural analysis of a thin insulating layer formed on the surface of a conductive material.
Furthermore, the above-mentioned device and means allow observation of a medium with low electric power without damaging it since they use a method of detecting a very weak current.
Because the STM can operate in the open air, extensive studies have been made for its application to various fields such as observation/evaluation and micropatterning of semiconductors or polymers on the atomic or molecular order (E. E. Ehrichs, Proceedings of 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, "89, S13-3), an information recording/reproduction apparatus, and the like, using the STM technique.
For example, upon applying the STM to an information recording/reproduction apparatus, the distal end portion of a tip of the STM preferably has a small radius of curvature to attain a high recording density. At the same time, in view of improvement in function, especially, an increase in speed of the recording/reproduction system, it is proposed to simultaneously drive a large number of probes (multi-tip structure). For this purpose, however, tips having uniform characteristics must be manufactured on a single substrate.
An atomic force microscope (hereinafter abbreviated as an "AFM") can measure a three-dimensional pattern image on the specimen surface, irrespective of whether conductors or insulators, since it can detect a repulsive force or an attractive force acting on the surface of a substrate.
The AFM uses a micro-tip formed on the free end of a thin-film cantilever. As in the STM, in order to increase resolution of the AFM, the distal end portion of the tip is required to have a small radius of curvature.
Furthermore, there is available a multi-functional microscope known as a composite scanning atomic force/tunneling microscope (AFM/STM) for conducting AFM and STM observation on a single microscope.
According to this microscope, the probe used in AFM comprises a cantilever and a tip held by this cantilever, and it is possible to detect current flowing between the tip and a sample by using a conductive tip.
In an ordinary usage, current is detected by applying bias between the tip and the sample upon operation of AFM, thereby making it possible to simultaneously obtain a surface irregularity image and a tunneling current distribution image with the same tip.
In this composite machine, also, an information recording/reproduction apparatus for writing information to be recorded in a local area is considered by the utilization of the accessibility of a tip to the sample surface at an atomic level. In this case, use of a plurality of probes is required for increasing the write or read speed.
The magnetic force microscope (MFM) is to measure the leak magnetic field distribution two-dimensionally and three-dimensionally in a non-destructive manner by detecting a force acting between the sample of a magnetic material such as a magnetic recording medium or a magnetic head with the use of a spring-acted probe comprising a magnetic material. A probe for MFM comprises a micro-tip having a sharp distal end portion and a cantilever, in which the micro-tip or the micro-tip and the cantilever has or have a magnetic layer. Apart from the structural evaluation of the sample, applicability of MFM to an information storing apparatus which records information directly in a magnetic recording medium using a produced magnetic field of the tip is studied (T. Okubo et al., IEEE Trans. Magn. MAG-27(6), pp. 5286-5288, 1991). When considering application to structural evaluation or to an information recording/reproduction apparatus, the radius of curvature of the distal end portion of the tip of MFM should preferably be small to achieve a high resolution and a high recording density.
As a conventional method of forming the foregoing micro-tip formed by anisotropic etching of monocrystalline silicon using a semiconductor manufacturing process is known (U.S. Pat. No. 5,221,415).
In the method of forming a micro-tip, as shown in FIG. 1, a pit 518 is formed by anisotropic etching on a silicon wafer 514 coated with silicon dioxide masks 510 and 512, and is used as a female mold of a tip. After the silicon dioxide masks 510 and 512 are removed, the two surfaces of the wafer 514 are coated with silicon nitride layers 520 and 521 to form a pyramid-shaped pit 522 which is to serve as a cantilever and a micro-tip. After the pit 522 is patterned into a cantilever shape, the silicon nitride layer 521 on the rear surface is removed. Then, a glass plate 530 with a saw-cut 534 and a Cr layer 532 is joined to the silicon nitride layer 520. Thereafter, the silicon wafer 514 is removed by etching, thus manufacturing a probe which is transferred to a mounting block 540 and is constituted by the silicon nitride tip and the cantilever. Finally, a metal film 542 serving as a reflection film for an optical lever type AFM is formed.
When this is used for MFM, it suffices to form a magnetic layer 543 by the use of the vacuum vapor deposition method on the surface of the foregoing probe. Similar examples in which a magnetic layer is formed by the vacuum vapor deposition method on the surface of a probe comprising silicon dioxide as formed on Si include an MFM proposed by A. Kikukawa et al. (Appl. Phys. Lett. Vol. 61 (21), Nov. 23, 1992, pp. 2067-2069), and that proposed by Hosaka et al. (Proc. 1992 Precision Eng. Society Fall Conf., H22, pp. 277-278).
Also, the following methods are available. That is, in another manufacturing method, as shown in (a) in FIG. 2, a thin film layer 202 on a silicon substrate 201 is patterned into a circular shape, the substrate 201 is etched using the patterned thin film layer as a mask, and a tip 203 is formed by utilizing side etching (O. Wolter, et al., "Micromachined silicon sensors for scanning force microscopy," J. Vac. Sci. Technol. B9(2), Mar/Apr., 1991, pp. 1353-1357). In still another manufacturing method, as shown in (b) in FIG. 2, a conductive material 207 is obliquely deposited onto a substrate 204 via a reverse-tapered resist aperture portion 206 of a resist film 205 while rotating the substrate 204, and is lifted off, thereby forming a tip 208 (C. A. Spindt, et al., "Physical properties of thin film field emission cathode with molybdenum cones," J. Appl. Phys., 47, 1976, pp. 5248-5263).
There is also available a probe for detecting magnetic force, formed by this method (K. Yanagisawa, et al., "Magnetic Micro-Actuator," Proceedings IEEE Micro Electro Mechanical Systems, 1991, pp. 120-123). It is now possible to form a probe having desired properties of those materials including a magnetic material having a large coercive force to which any magnetizing direction can be imparted through magnetization, and a magnetic material having a small coercive force in contrast in which the magnetizing direction can be directed in the measuring magnetic field direction, by the application of these methods, as probes for detecting magnetic force applicable in MFM and the like.
However, the conventional micro-tip manufacturing method suffers the following problems:
For example, the conventional micro-tip manufacturing method shown in FIG. 1 suffers the following problems:
(1) Since the silicon substrate used as a female mold of a tip is removed by etching in a subsequent process, it cannot be reused, resulting in a lower productivity and a higher manufacturing cost.
(2) Since the silicon substrate used as a female mold of a tip is etched, deterioration of the tip material and the tip shape on the probe surface due to an etchant, and contamination from the etchant occur.
(3) When a tip for the STM is manufactured by coating the tip surface with a conductive material, since the tip is formed to have a sharp distal end, it is not easy to form a coating of the conductive material. When the tip surface is coated with the conductive material, grain clots of the conductive film appear, and it is difficult to control the grain clots with a high reproducibility.
(4) Furthermore, when a micro-tip is formed on a thin-film cantilever, a reflection film is formed on the entire rear surface of a probe in an AFM, and the cantilever warps due to the film stress of the reflection film. Conventional films of SiO.sub.2, SiN, SiC and C formed by vacuum vapor deposition or the CVD method are polycrystalline or amorphous and contain considerable internal stress, resulting in a problem of occurrence of warp in the level itself. When there are fluctuations in the lever caused by warp, use of a plurality of probes leads to different loads for the individual probes relative to the recording medium, and depending upon the extent of load, there may occur a decrease in resolution or destruction of the recording medium or the tip.
When causing a thick substrate such as an Si one to hold a portion of a thin-film lever comprising SiO.sub.2 or SiN, a stress is produced at the laminating portion of the substrate and the film, and the stress concentrates particularly on the base portion of the lever. If the lever is operated repeatedly, therefore, breakage is caused at this portion.
Furthermore, when information is recorded through application of voltage onto the recording medium by means of a probe comprising the cantilever and the substrate holding it, the entire surface of which is coated with a conductive material, the cantilever and the entire surface of the substrate holding it serve as an electrode. A floating capacity is therefore produced between this electrode and the recording medium, thus resulting in a problem of a delay in the application time of voltage.
(5) The conventional method of manufacturing a micro-probe for detecting magnetic force comprises the step of forming a magnetic material layer on the entire surface of a probe by the vacuum vapor depositing method. When the cantilever is brought closer to the sample, therefore, not only the probe but also the magnetic material layer formed on the cantilever may be susceptible to leak magnetic field, and this causes an increase in noise of the detection signal.
For the purpose of inhibiting warp of the cantilever caused by the film stress of the magnetic material layer resulting from formation of the magnetic material layer over the entire surface of the cantilever, the magnetic material layer should be a thin film having a thickness within a range of from several to several tens of nm, thus leading to a lower detection sensitivity of magnetic force.
When the surface of the probe is coated with the magnetic material layer to serve as a probe for MFM, the distal end portion of the probe is formed into a sharp shape. This makes it difficult to coat it with a magnetic material, and when actually coating same, grain clots of the formed magnetic material layer appear, and it is difficult to control the grain clots with a high reproducibility.
The conventional micro-tip manufacturing method shown in FIG. 2 suffers the following problem:
(6) Strict process management is required to make constant the etching condition for silicon upon forming a tip, the patterning condition for a resist, the deposition condition of a conductive material, and it is difficult to maintain accurate shapes such as the heights, the radii of curvature of distal ends, and the like of a plurality of micro-tips to be formed.
(7) Particularly when a cantilever-shaped probe coated with a conductive material is used as an STM probe, the distal end portion of the tip is formed into a sharp shape, so that it is difficult to coat it, and in STM handling a weak current called tunneling current, it is difficult to obtain stable properties.